High-voltage power supply with temperature-dependent zener in a microfilm printer device

ABSTRACT

A highly reliable high-voltage power supply, having a temperature stability of 0.01 of 1 percent per *C., in a microfilm printer device, is described; it comprises, in combination with a high-voltage DC power source connected through a series resistor to an output line, a regulator device connected between the output line and the output of an operational amplifier, a feedback resistor connected between the output and the summing junction of the amplifier, and a constant-voltage reference of voltage lower than that at the summing junction connected through a resistor to the summing junction, the output of the regulator device being subject to variation with change in temperature and the amplifier, constant-voltage reference, and their resistors being adapted to vary the voltage of the amplifier output oppositely to the variation of the output of the regulator device, thereby to maintain the output line at a constant voltage.

United States Patent Inventor Asger T. Nielsen San Diego, Calif. 38,165

May 18, I970 Nov. 30, 1971 Compufoto, Inc.

Wellesley Hills, Mass.

App]. No. Filed Patented Assignee HIGH-VOLTAGE POWER SUPPLY WITH TEMPERATURE DEPENDENT ZENER IN A MICROFILM PRINTER DEVICE 9 Claims, 1 Drawing Fig.

References Cited UNITED STATES PATENTS 5/1964 Harries 323/22 Z COMPUTER Primary ExaminerGerald Goldberg Anamey-John Noel Williams ABSTRACT: A highly reliable high-voltage power supply, having a temperature stability of 0.01 of 1 percent per C., in a microfilm printer device, is described; it comprises, in combination with a high-voltage DC power source connected through a series resistor to an output line, a regulator device connected between the output line and the output ofan operational amplifier, a feedback resistor connected between the output and the summing junction of the amplifier, and a constant-voltage reference of voltage lower than that at the summing junction connected through a resistor to the summing junction, the output of the regulator device being subject to variation with change in temperature and the amplifier, constant-voltage reference, and their resistors being adapted to vary the voltage of the amplifier output oppositely to the variation of the output of the regulator device, thereby to maintain the output line at a constant voltage.

PATENTEU NUVBO I97! COMPUTER Objects of this invention are to solve certain computer output microfilm problems stemming from variable-power supply voltage and in general to provide a low-cost, high-accuracy, high-voltage power supply for electronic devices that require extreme accuracy of voltage level.

Another object is to provide a power supply that will exhibit high reliability.

Typical basic parameters for such a power supply are a voltage rating of 10,000 volts DC at a current rating of 10-20 microamperes, with a regulation in the order of l/ 10 of 1 percent versus line variations and 1/10 of 1 percent versus load variations. The DC level should have a ripple content not exceeding l/l of 1 percent. A further important factor is stability with time: and with temperature. The output voltage level should be stable within 1/ of 1 percent for a period of not less than 8 hours, and stable within approximately of 1 percent over a period of several weeks.

This invention features the use of a low-cost, off-the-shelf power source, a series resistor and a constant high-voltage regulator connected to the output of an operational amplifier. A feedback resistor from the connection of the series resistor and the constant voltage regulator directs a low current to the summing junction of the operational amplifier. Another current of equal magnitude is drawn from the summing junction to a second constant (low) voltage source. Due to the nature of a closed-loop operational amplifier the output of the amplifier will adjust itself at a voltage level that will cause the amplifier to be perfectly balanced. This takes place only when the two currents mentioned above are equal. Since the current that flows through the feedback resistor mentioned above creates a voltage drop across that resistor, it is now possible for the system to maintain a constant voltage at the junction of the feedback resistor, the series resistor and the constant highvoltage regulator despite variations in the high-voltage regulator, due e.g., to change of temperature.

Referring to sole FIGURE, one preferred embodiment of the invention is shown. It consists of high-voltage DC power supply 2, a series resistor R,, zener diode Z, connected to output 3, feedback resistor R operational amplifier 4, having a summing junction 5, a resistor R connected between the summing junction and reference zener diode 2 which in turn has a series resistor R, connected to a negative supply V. Two additional zener diodes, Z and 2,, are provided as protection for the output stage of the operational amplifier.

.Typical values are as follows: the DC power supply 2 supplies 15,000 volts at 1% milliamperes. lt receives its input from 100-volt 60-112. supply. The value of R, is chosen such that a zener current through 2, is approximately 250 microamperes. Together with a feedback current through R, of approximately 200 microamperes. R, typically has a value in the order of 10 megohms. The value of R is 50 megohms and the value of R is 31,000 ohms. 2:, the second constantvoltage regulator or reference, is a zener diode with an extremely low temperature coefficient, typically with a voltage rating of 6.2 volts. The most stable operation of 2 is obtained when it is exposed to the ideal zener current of 2% milliamperes.

This requires R, to have a value of 1,760 ohms if the voltage of the negative supply is volts. The value of each of the two zener diodes, 2,, and 2,, is 140 volts. The manner in which Z and 2 are connected protects the output stage of the operational amplifier at both 140 and +140 volts.

DC power supply 2 will supply 15 kilovolts, well above the output of the power supply which is 10 kilovolts at the junction 3 between R, and R The zener diode 2, has a rating of 10 kilovolts but its function is dependent to some degree on temperature, being much more variable than the 6.2-volt zener diode Z mentioned above. The specification for presently available high-voltage zener diodes calls for a temperature dependency of :l/ 10 of 1 percent for each 1 C.

change in temperature. During typical operation, the zener diode Z, might be exposed to a temperature change of :10, resulting in a total change of zener voltage of :1 percent. 1f the total range is 10,000 volts, this variation in the output would amount to 1100 volts. If this variation were permitted to appear in the output 3 of the overall power supply, the power supply would be unusable for many applications such as high-speed computer output microfilm devices using CRTs.

However, with the present circuit if the temperature of the system changes by 10 and the voltage rating of the zener diode Z, changes by volts, the current through R, will change. This will cause the voltage at the summing junction 5 to differ from the voltage at the noninverting positive input to the operational amplifier. The result of this difference is a change in the output of the operational amplifier which is out ofphase with the input change. In other words, if the input changed in a positive direction the output would change in a negative direction. This would cause the lower end of the zener diode Z, to become more negative, and accordingly, the upper end of the zener diode will also become more negative than it was before, thus counteracting the voltage increase caused by a change in temperature of the zener diodeZ,. (lf, instead, the temperature change caused a decrease in voltage across 2,, the circuit would serve to raise the voltage of the lower end of zener diode Z,, and again maintain the output 3 at constant voltage).

The operational amplifier has an output voltage range which far exceeds the maximum possible variation of 'the zener diode Z, and can therefore overcome any variation in the voltage across 2,. The open loop gain of the operational amplifier is in the order of 20-30 thousand times while the closed loop gain due to the ratio of R to R is in the order of 1,700 times. This results in a gain advantage of approximately 12 times. We can therefore expect that the stability of the power supply will be approximately 12 times better than that of the zener diode, 2,. In other words, we have obtained a stability of 0.01 of 1 percent per 1 C. as compared to the inherent stability of the zener diode of 1/ 10 of 1 percent per C. Components that can be used successfully in a system of this description are as follows: the 15,000-volt DC power supply may be one manufactured by Plastic Capacitors, lnc., of Chicago, 111., Part No. HV ISO-152M. [t is an off-the-shelf item which has a proven record of reliability. Expected lifetime is on the order of 40,000 hours. A suitable zener diode Z, is one manufactured by General Semiconductor Industries, lnc., of Tempe, Ariz. It carries Part No. GS 007 G2 91231A. The operational amplifier 4 may be of conventional type provided that its output voltage capability is on the order of :1 50-200 volts.

1n the preferred embodiment the power supply just described is combined with other devices to provide a microfilm printer device. Thus computer 19 controls the magnetic beam deflection and the blanking circuits of CRT 14 while the power supply just described controls the electron acceleration velocity. The electron beam striking the phosphor of the tube traces desired alphanumeric characters or graphics on the tube which is microfilm photographed by camera 12. The light from the CRT passes through a beam combiner 18 on the the way to the camera 12. At the combiner this light is combined with light that has been projected through formsoverlay transparency or slide 16, to produce e.g., a complete filled-in business form for photographing.

It is common practice to utilize cathode-ray tubes with highaccuracy magnetic deflection. The high-voltage power supply providing the electrostatic acceleration field in the cathoderay tube must therefore be extremely stable, as otherwise a reduction in format size of the display will result. On visual displays this can quite easily be tolerated, but on a photographic display where a business form is superimposed on the computer-generated information and where proper alignment between the two displays is essential, variations in display size cannot be tolerated. The characters displayed on the cathoderay tube are about 0.022 inch in size. A variation in the lO-kv.

voltage of the power supply could very easily cause these characters to overlap completely lines displayed by the form slide 16; as a result. the characters would appear, not in the intended column, but in a neighboring one. The computer output microfilm device just described avoids this problem.

What is claimed is:

1. In a computer output microfilm device having a cathoderay tube, a high-voltage DC power supply having its output line connected to power the electron accelerator for said tube to produce a beam tracing figures to be microphotographed, said supply comprising a high-voltage DC power source connected through a series resistor to said output line, a regulator device connected between said output line and the output of an operational amplifier, a feedback resistor connected between said output line and the summing junction of said operational amplifier, and a constant voltage source of voltage lower than that at the summing junction connected through a resistor to said summing junction, the output of said regulator device being subject to variation with change in temperature, and, together, the amplifier, said constant voltage source and said resistors adapted to cooperate to vary the voltage of the amplifier output opposite to variation of the output of said regulator device thereby to maintain said output line at constant voltage.

2. The computer output microfilm device ofclaim l in combination with means for combining in a single photograph the CRT display and additional prearranged matter.

3. The combination of claim 1 wherein said power source is adapted to produce a voltage output on the order of 10,000 volts DC at said output line and said regulator device comprises a zener diode having a rating on the order of 10,000 volts.

4. The combination of claim 3 wherein said constant voltage source includes a zener diode having a rating on the order of 6 volts.

5. A high-voltage DC power supply comprising the combination of a high-voltage DC power source connected through a series resistor to an output line, a regulator device connected between said output line and the output of an operational amplifier, a feedback resistor connected between said output line and the summing junction of said operational amplifier, and a constant voltage source of voltage lower than that at the summing junction connected through a resistor to said summing junction the output of said regulator device being subject to variation with change in temperature, and, together, the amplifier, said constant voltage source and said resistors adapted to cooperate to vary the voltage of the amplifier output opposite to variation of the output of said regulator device thereby to maintain said output line at constant voltage.

6. The combination of claim 5 wherein said power source is adapted to produce a voltage output on the order of l0,000 volts DC at said output line and said regulator device comprises a zener diode having a rating on the order of l0,000 volts.

7. The combination of claim 6 wherein said constant voltage source includes a zener diode having a rating on the order of 6 volts.

8. The combination of claim 7 wherein said resistors are selected in reference to the other components to establish substantially the ideal current of 2 k milliamperes through said constant voltage source.

9. The combination of claim 8 wherein said operational amplifier has an open-loop gain on the order of 20,000 times and a closed loop gain on the order of 1,500 times and has an output voltage on the order oftl 50 volts. 

1. In a computer output microfilm device having a cathode-ray tube, a high-voltage DC power supply having its output line connected to power the electron accelerator for said tube to produce a beam tracing figures to be microphotographed, said supply comprising a high-voltage DC power source connected through a series resistor to said output line, a regulator device connected between said output line and the output of an operational amplifier, a feedback resistor connected between said output line and the summing junction of said operational amplifier, and a constant voltage source of voltage lower than that at the summing junction connected through a resistor to said summing junction, the output of said regulator device being subject to variation with change in temperature, and, together, the amplifier, said constant voltage source and said resistors adapted to cooperate to vary the voltage of the amplifier output opposite to variAtion of the output of said regulator device thereby to maintain said output line at constant voltage.
 2. The computer output microfilm device of claim 1 in combination with means for combining in a single photograph the CRT display and additional prearranged matter.
 3. The combination of claim 1 wherein said power source is adapted to produce a voltage output on the order of 10,000 volts DC at said output line and said regulator device comprises a zener diode having a rating on the order of 10,000 volts.
 4. The combination of claim 3 wherein said constant voltage source includes a zener diode having a rating on the order of 6 volts.
 5. A high-voltage DC power supply comprising the combination of a high-voltage DC power source connected through a series resistor to an output line, a regulator device connected between said output line and the output of an operational amplifier, a feedback resistor connected between said output line and the summing junction of said operational amplifier, and a constant voltage source of voltage lower than that at the summing junction connected through a resistor to said summing junction the output of said regulator device being subject to variation with change in temperature, and, together, the amplifier, said constant voltage source and said resistors adapted to cooperate to vary the voltage of the amplifier output opposite to variation of the output of said regulator device thereby to maintain said output line at constant voltage.
 6. The combination of claim 5 wherein said power source is adapted to produce a voltage output on the order of 10,000 volts DC at said output line and said regulator device comprises a zener diode having a rating on the order of 10,000 volts.
 7. The combination of claim 6 wherein said constant voltage source includes a zener diode having a rating on the order of 6 volts.
 8. The combination of claim 7 wherein said resistors are selected in reference to the other components to establish substantially the ideal current of 2 1/2 milliamperes through said constant voltage source.
 9. The combination of claim 8 wherein said operational amplifier has an open-loop gain on the order of 20,000 times and a closed loop gain on the order of 1,500 times and has an output voltage on the order of + or - 150 volts. 